It's a bit more complicated than that.

If you register your app to one of those 7 services they identified, than your app will not be killed by iPhone OS 4, just suspended in RAM and the OS will take care of waking it up to handle incoming input (that's what the services are for; for example Skype incoming TCP socket is handled by the system that will know when to wake Skype app up to let it handle incoming messages or calls).
If you do not register to one of those services, than you app, again, will not be killed by iPhone OS 4, but rather put in suspended mode again and only killed if RAM is needed (like Android thus).

In suspended mode you get no CPU time at all, battery consumption will not be touched and if the user switches back to your app, it'll be there.

Of course the app will get a message saying it's going to be suspended so that it has time to save the state in case it would be later killed to free RAM.

This is different than N900 (pure multitasking) and Android (app continues to run in background, but can get killed if it is less used than the others when more RAM is required).

Both N900 and Android provide better multitasking experiences, but this can and will slow down the system in case there are too many and affect battery life.

But, again, I'd rather control this myself than letting Steve do it for me.

What N900 COULD do is to kill/suspend last used application WHEN the load is too high (and thus is badly affecting the user experience with the phone).
This would mean the best, to me, multitasking experience.

I suspect a similar behavior is already present in Maemo but only when RAM is going low. No one remembers the "Zzz" symbol that appeared on N800 running apps when free RAM was low?